A well known mechanical lash adjuster of a rocker arm type mechanical lash adjuster has a rocker arm operably connected to a valve stem of an intake/exhaust valve installed in the cylinder head of an automobile engine so that the valve clearance is automatically adjusted by extension and retraction of the lash adjuster which serves as a fulcrum of the rocker arm. (See for example Patent Documents 1 and 2, and non-patent document 1 listed below.)
This type of mechanical lash adjuster has: a cylindrical housing formed with an internal female thread; a pivot member formed with a male thread on its exterior, with a lower portion of the pivot member retained in the housing; and a plunger spring (compression coil spring) biasing the pivot member upward towards an upper rocker arm, wherein the male and female threads are engaged together to form buttress threads. In this mechanical lash adjuster, the thread angles (lead and flank angles of the buttress threads) are set such that the buttress threads undergo relative sliding rotation to extend the pivot member to automatically adjust the valve clearance under an axial load applied thereto, but otherwise become unrotatable not to retract the pivot member by the friction between the two engaging threads. Such suppression of the rotation of threads by the friction between them will be hereinafter referred to as independence of the threads.